The continuing advancement in microelectronic devices has led to tremendous growth in the development of smaller circuit boards and processors having greater speed and computing capabilities. Due to these new developments, the amount of heat generated per unit area in these circuit boards has also increased. As a result, overheating is one of the major causes of electronic failures in microelectronic devices.
In order to combat overheating, heat sinks are employed to remove heat from electronic devices. But even the best heat sinks cannot dissipate heat efficiently until there is an intimate contact between the processor unit and the heat sink. For this purpose thermal interface materials (“TIM”) are used. These thermal interface materials flow and conform to the surface topologies of solids in contact. Thus, permitting intimate contact. Current thermal interface materials are made of polymer gels, pads or liquids. Some of these polymer liquids may even have ceramic or metal particles to enhance their conductivity, or be based on phase change materials. However even for the most efficient systems, heat transfer across interface is the bottleneck to enhance heat flow. These materials attempt to squeeze air pockets out of the system to enhance heat transfer efficiency between the processor unit and the heat sink. To enhance the heat transfer efficiency significant fraction of conductive particles such as silver may be added to current thermal interface materials.
Yet, there are significant problems associated with current thermal interface materials. Being liquid in nature, these materials tend to leak out over a period of time. This leads to lowered efficiency with respect to time and contamination of the circuit board. In cases where the conductive particles are added, contamination may lead to short circuiting.
As stated above, commercially available thermal interface materials can be categorized under the following categories: 1) thermal greases, 2) thermal pads, and 3) conductive adhesives, 3a) silver epoxies, and 3b) acrylate adhesives with ceramic particles dispersed within them. thermal interface materials based on thermal grease may be silicon based polymers or some other polymers with suitable viscosity and melting points. Being organic in nature, these types of thermal interface materials have high thermal resistance. Conductive fillers are sometimes added to enhance their thermal conductivity. These conductive fillers may reduce overall bulk resistance, but heat transfer across interfaces may not be very efficient since heat transfer may happen via phonons. The other main drawback of these thermal greases is that their low viscosity at higher temperatures may cause leakage. Thus, their efficiency may reduce over a period of time. Such thermal greases are also not suitable for cases where voids are large.
In addition, phase change materials are available as thermal pads. These thermal pads can change their physical characteristics with temperature. Usually phase change materials used for thermal interface materials may change from solid to liquid form at around 45° C.-50° C. These thermal pads are easier to handle than thermal greases. After installation these thermal pads may adhere to the components. Therefore, removing them from the components may cause some damage. Phase change materials are generally put in between heat sinks and a processor, the heat sink being clamped into place. The disadvantage in such cases is that these materials have different physical properties, e.g. modulus and flow, at different installation and working temperatures. Thus, it may not be working at its highest efficiency. Silver filled epoxies may have very high thermal conductance, but these are rigid substances. If the two bonding surfaces have different thermal expansion coefficients, such rigid areas at the interface may cause damage. Junction material with room for expansion and contraction is desirable. FIG. 1 summarizes some of the commercially available thermal interface materials. Therefore, a need exists for a more efficient, robust material for thermal and electrical conductivity. The thermal conductance of commercially available thermal interface materials is shown in FIG. 1, where it is noted that carbon nanotube arrays have been found to provide significantly higher thermal conductance than other commercial materials.
In the current design of thermal interface materials, electronic and radiative heat transfer is used as a guideline in their design. Phononic heat transfer may be ballistic within the bulk of a crystalline solid, but at interfaces phonons may get reflected thus causing low efficiency of heat transfer across an interface. Electronic energy can tunnel across the interfaces in metallic systems. Another efficient way for heat flow across interfaces is radiation.
There have been attempts toward synthesizing carbon nanotube based thermal interface materials. However all the previous attempts had one or more of following shortcomings: 1) The films were not free standing: As-grown carbon nanotubes on copper or other metallic/silicon substrate have been tested for their thermal resistance. For commercial use such systems cannot be used. It is required that the material be available in form of free standing thermal pads; 2) Non-compliable geometries: In many cases the array was not tailored such that it could allow maximum area of contact with the adhering surfaces. Such high area of intimate contact is very important to achieve low thermal resistance across the interfaces; 3) Polymer impregnated systems: In the previous systems the carbon nanotubes have been dispersed on polymeric materials or even in cases where aligned nanotubes were impregnated with polymer based systems, the array was completely embedded in polymers. Such geometries not only lead to higher modulus of the whole system (this less intimate contact, leading to higher thermal resistance), but it will also lead to lower thermal conductivity of the whole system; and 4) Low mechanical stability: For commercial use of the material, it is required that they have high mechanical stability. Such high mechanical stability will allow easy handling, packaging, transportation and re-usability of these systems. Based on the above, there is a need for the development of novel structures of free standing metallic carbon nanotube arrays with conformable geometry.
Further limitations and disadvantages of conventional, traditional, and proposed approaches will become apparent to one of skill in the art, through comparison of such systems and methods with the present invention as set forth in the remainder of the present application with reference to the drawings.